166 



PRACTICAL PHYSIOLOGY 



CHAPTER XLI. (Advanced Demonstrations). 

 INTRACARDIAC PRESSURE. BLOOD FLOW. 



Intracardiac Pressure. Owing to inertia the mercurial manometer 

 is unable to respond to the rapid changes of intracardiac pressure. The 

 pulse curves obtained by the mercurial manometer are also distorted by 

 the swings due to the momentum of the mass. To record the changes 

 of intracardiac pressure an instrument must be contrived which is able 

 to follow a change of pressure equal to 1500 mm. Hg per second. 



FIG. 165. Hiirthles spring manometer. 



FIG. 166. Sphygmoscope. 



Hiirthle's spring manometer consists of a small tambour, 5*5 in 

 diameter, covered with rubber membrane. A button attached to the 

 membrane works against a steel spring. The movement of the spring 

 is magnified by a light lever. Inertia is proportional to the mass and 

 the square of the velocity. By making the tambour very small and 

 the lever very light the errors due to the inertia of the fluid and lever 

 are reduced to a minimum. 



FIG. 167. Arterial pressure recorded by a spring manometer. Effect of weak 

 excitation of the vagus during the period marked by the signal m. (Dubois). 



The sphygmoscope is an equally good instrument. One end of a 

 rubber finger-stall is drawn over the end of a rubber cork. The cork is 

 inserted into a short piece of wide tube. A glass tube passes through 

 this cork into the small air-space which is left at the top of the finger- 

 stall. The other end of the wide tube is closed by a rubber cork. A 



